The calculator is slower than you: when to reach for it, and when not to.
Build percentages from moves you can do in your head: 10%, 5%, 1% and 25%. Need 35%? That is 25% plus 10%. Need 3%? That is 1% three times. For round figures the ladder is faster and less error-prone than typing — the calculator is for ugly division and multi-digit multiplication, not for halving something.
Estimate to one significant figure first, then compute. The on-screen calculator has no order of operations, no fractions and no backspace — it takes one operation at a time, so chain the steps yourself and keep one hand on the keypad with Num Lock on. If the result lands far from your estimate, you have mistyped or misread — check the data before trusting the keypad.
Percentage change is always change divided by the original, not the new value: 120 to 150 is 30 over 120, a 25% rise. And sequential changes multiply rather than add: a 10% rise then a 10% fall is ×1.1 then ×0.9 — 99% of where you started, not 100%.